OK, Reboog711 put me on the right track, I needed to modify my factory definition to include $http, I had previously tried to do this, but mistakenly put it together like this:
'$rootScope, $http'
Don't do this, it's bad and wrong! But Reboog711's answer also gave me errors because I think you can't use $scope in the way we both thought.
The correct (or working) solution was:
app.factory('quizService', ['$rootScope', '$http', function ($rootScope, $http) {
I hope this helps other newcomers to Angular. Many thanks to all who replied with their comments. I really respect this community for its great attitude to helping others :)